The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are illustrated in FIG. 1. They are seed layer 11 (lying on lower conductive lead 10) on which is antiferromagnetic layer 12 whose purpose is to act as a pinning agent for a magnetically pinned layer. The latter is a synthetic antiferromagnet formed by sandwiching antiferromagnetic coupling layer 14 between two antiparallel ferromagnetic layers 13 (AP2) and 15 (AP1).
Next is a non-magnetic spacer layer 16 on which is low coercivity (free) ferromagnetic layer 17. A contacting layer such as lead 18 lies atop free layer 17. When free layer 17 is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field.
If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8-20%.
GMR devices may be designed so as to measure the resistance of the free layer for current flowing parallel to its two surfaces. This is referred to as a CIP (current in plane) device.
Instead of being a single layer, free layers that are laminates of several layers have begun to be used in magnetic recording heads. For example, Co90Fe10/Ni80Fe20, A typical composite free layer usually consists of two magnetic layers, a first free layer (FL1) and second free layer (FL2), which are directly magnetically coupled to one another. FL1 (usually Co-rich alloys) provides strong spin dependent scattering, while FL2 (usually permalloy-type (NiFe) material) provides magnetic softness (i.e. low coercivity).
When compared with a free layer of only CoFe, a composite free layer has the following advantages: 1) Better magnetic softness can reduce noise and enhance the sensitivity of GMR sensor. 2) Magnetostriction can be easily adjusted by changing the thickness ratio of Ni80Fe20 to Co90Fe10. However, a major drawback of composite free layers of the current and prior art is their low dR and dR/R in a CIP configuration because Ni80Fe20, with relatively low spin polarization and low resistivity, significantly contributes to shunting effects while top specular (or spin filter) schemes, such as CoFe\Cu\Oxide or CoFe\Oxide, cannot be applied in this case.
A routine search of the prior art found the following references to be of interest:
In U.S. Pat. Nos. 6,614,630 and 6,517,896 (Horng et al) show conventional CoFe/NiFe free layers. Gill teaches alternating CoFe and NiFe films to form the free layer in U.S. Pat. No. 6,466,417. In U.S. Pat. No. 6,038,107 Pinarbasi discloses a composite Co/NiFe free layer while Den discloses FeNi in the ferromagnetic layer in U.S. Pat. No. 6,611,034.
Tanaka et al. describe a Co70Fe15Ni15 free layer having a ratio of 70:15:15 U.S. Pat. No. 6,608,740. In U.S. Pat. No. 6,123,780, Kanai et al) show a FeNi/CoFeB free layer but give no details on the Fe composition of the layer. In U.S. Pat. No. 5,896,252, Kanai describes a spin valve that includes two sub-layers and, in U.S. Pat. No. 6,352,621, Saito et al. disclose a FeNi free layer but give no details on the Fe composition of the layer.